Icirrus City's appearance depends on the seasons. In Winter, there is snow everywhere that you can walk on, which will allow you to reach parts of the town that you can't reach in other seasons.
There are puddles on the ground here where you can encounter Stunfisk and Shelmet if you walk on them.
Wye, Aha, and Ditoh's house
In one house here, the members of the household will give you a quiz that you need to answer using the game's word picker. If you get the correct answer, they give you an Antidote. If you give an incorrect answer, they give you a Parlyz Heal. You can do this once per day.
Pokémon Fan Club
Another feature of Icirrus City is the pokémon fan club. If you have a pokémon that you caught at a low level and have raised to a high level, the chairman will give you items. This will be either an Exp. Share, Cleanse Tag, or King's Rock, depending on how many levels you have raised your pokémon through.
Ex-Team Rocket Grunt's House
In the Winter, a mound of snow in the southern part of Icirrus City will allow you to reach the previously inaccessible Ex-Team Rocket Grunt's house. This is the same grunt who stole the machine part from the Kanto Power Plant and has a weird way of speaking. He's happy that he has a family now. His wife will give you a RageCandyBar. You can use this RageCandyBar to catch a Darmanitan that has the hidden ability Zen Mode near the Relic Castle entrance.